1. Field of the Invention
The present invention relates generally to communication systems. The invention solves the problem of developing a network protocol over a propagation channel that is subject to frequent intensity fades due to turbulence effects, e.g., free space optical (FSO) communications channels. The invention addresses the problem of measuring the bit error rate BER through fading channels using standard bit error rate testers BERT's. The invention involves a method of characterizing data transmission through a free-space optical channel. In other aspects the invention concerns a receiver and hardware, firmware and software designed to perform the method.
2. Description of the Related Art
Developing a network protocol over a propagation channel requires a detailed knowledge of the characteristics of the channel. As a function of time, the channel may be impaired by noise if the channel experiences power loss due to fades as a result of a variety of physical effects. The channel may also experience impairments due to time-varying dispersions. While the measurement of a single impairment may be possible, such as recording Signal-to-Noise ratio as a function of time, the combined effects of all impairments on the communication system can really only be measured by recording the Bit Error Ration (BER). As is well known, the BER is the system level characterization that reveals the effectiveness of the channel for communications. This ratio can be measured by sending known information at the transmitter, and comparing the information received after going through the propagation channel to expected information. The data sent over the channel may be packet based or some fixed pseudorandom bit stream. However, a channel that is time varying and experiences deep fades where complete data loss occurs could rapidly have the measurement results dominated by the fades, even if they do no occur frequently. For example, free Space Optical (FSO) communications channels can exhibit high percentage availability, yet are subject to frequent intensity fades due to turbulence effects.
For gigabit class links, tremendous amounts of data can still be transported through a fading channel, but an efficient network protocol is required to overcome the effects of fades. However, developing a network protocol over a propagation channel requires a detailed knowledge of the characteristics of the channel. As a function of time, the channel may be impaired by noise if the channel experiences power loss due to fades that result from a variety of physical effects. The bit error ratio is the system-level characterization that reveals the effectiveness of the channel for communications. However, a problem with available bit error rate testers BERT's is that they are designed for non-faded signals and their results are dominated by fades if the fades occur more rapidly than the instrument can be polled for data. Consequently, the measurement of bit error rate through fading channels using standard bit error rate (BER) detectors/testers is that they fail to successfully characterize the channel for data transmission. Furthermore, standard BER detectors do not need to acquire quickly, with the longer acquisition times guaranteeing pattern lock. This results in large penalties in faded channels. Still further, the timing accuracy of these commercial testing devices regarding their time to synchronize to the data and declare loss of synchronization when the signal is lost is not provided by manufacturers, and in some cases is quite an extensive amount of time. In a link that experiences frequent fades, these unknown parameters can significantly influence the measurement results. With propagation channels such as those provided by Free Space Optical (FSO) Communications, the data rate is many Gigabits per second. Consequently, the fades of the channel become the dominant feature of the link. Conventional BER detectors are designed for fiber systems with nearly constant link characteristics, where error tests are done over many hours to establish a degree of confidence for ultra-low bit error ratio floors of 1×10−12 and below. Free Space Optical (FSO) channels, however, have frequent fades, typically in the tens to hundreds per second which result in loss of meaningful statistics, as the Bit Error Ratio (BER) summary results are dominated by even a single fade event. Furthermore, polling the instrument is often limited in speed, so that attempting to capture results between fades are not successful.
Another drawback of standard BER detectors is that they fail to provide fade related impact statistics. That is, they fail to reveal channel dynamics. This is disadvantageous in that the understanding of channel dynamics can reveal optimal packet length strategies.
Accordingly, a need exists for a customized bit error rate (BER) tester to reveal the true statistics of a channel for proper network layer design. In particular, it is desirable to have a measurement device that can characterize a propagation channel without adopting a protocol that may be non-optimal for the link.